The present invention relates generally to the field of capacitors. More specifically, the present invention relates to electrolytic capacitors for use in medical devices (e.g., implantable medical devices) or other types of devices.
Since their earliest inception, there has been significant advancement in the field of body-implantable electronic medical devices. Today, such implantable devices include therapeutic and diagnostic devices, such as pacemakers, cardioverters, defibrillators, neural stimulators, drug administering devices, and the like for alleviating the adverse effects of various health ailments.
Implantable medical devices may utilize a capacitor to perform various functions. For example, if the implantable medical device is a defibrillator, one or more capacitors may be used to provide a therapeutic high voltage treatment to the patient.
One type of capacitor that may be used in such an application is an electrolytic or wet slug capacitor. Conventional wet slug capacitors may include a container formed from tantalum or a tantalum alloy that acts as the cathode for the electrolytic capacitor. An electrolyte (e.g., acid such as sulfuric acid) and an anode are provided within the container. In these types of capacitors, a native oxide may be formed on exposed surfaces.
Since the electrolyte is electrically conductive, a conductor-insulator-conductor structure including metal, oxide coating, and electrolyte is present at both the anode and the cathode. Each of these conductor-insulator-conductor structures is itself a capacitor (e.g., an anode capacitor and a cathode capacitor).
In the conventional wet slug capacitor, the anode capacitance is effectively electrically connected in series with the cathode capacitance. The amount of charge at the cathode and anode surfaces are substantially equal and of opposite sign. It should also be noted that the net capacitance of two capacitors connected in series is smaller than the smaller of the capacitances of the two capacitors. Because the oxide layer at the anode of a wet slug capacitor is usually much thicker than the thickness of the oxide layer at the cathode, the anode capacitance of a wet slug capacitor is generally smaller than the cathode capacitance.
The capacitance of a wet slug capacitor can be described using the following equation:
      C    Capacitor    =                    C        Cathode            ·              C        Anode                            C        Cathode            +              C        Anode            In general, it is desirable to increase the capacitance of the cathode to decrease the risk of forming hydrogen gas at the cathode and to make the capacitance of the anode more clearly observable. Although conventional wet slug capacitors having useful capacitances have been produced, there is a desire to increase the capacitance per unit area and capacitance per unit volume of the cathode coating material. Conventional cathode coating materials (e.g., tantalum), however, may provide a limited capacitance per unit area and limited capacitance per unit volume. For certain applications, it is desirable to provide a capacitor coating material that has a capacitance no less than approximately 10-20 milliFarads per square centimeter.
Accordingly, it is desirable to provide a capacitor that provides one or more of these or other advantageous features. Other features and advantages will be made apparent from the present description. The teachings disclosed extend to those embodiments that fall within the scope of the appended claims, regardless of whether they provide one or more of the aforementioned advantageous.